Conventional technologies of tunable lasers are confronted with technical difficulties and limitations. Specifically, tunable lasers which are implemented with mechanical or temperature tuning to change the cavity length to tune the wavelength do not provide stable and accurate turning in terms of power and frequency. More details of such technical limitations are more fully discussed in references such as “Stress induced tuning of a diode laser excited monolithic Nd:YAG laser,” by Adelbert Owyoung and Peter Asherick, in Opt. Lett. 12(12), 999-1001 (1987); and “Efficient, frequency stable laser diode pumped Nd:YAG laser,” by Bingkun Zhou, T. J. Kane, G. J. Dixon, and R. L. Byer, in Opt. Lett. 10(2), 62-64 (1985).
On the other hand, different approaches using tuning grating for frequency tuning of linewidth not very narrow, acoustic optical tuning filter (AOTF), or a Fabry Perot (FP) cavity can provide stable operation. However, these tuning methods cannot provide continuous tuning and accurate access of the wavelength due to the difficulties of hysterises. More details about the discussions of the hysterises are disclosed in several references. These references are: Hidemi Tsuchida, “Tunable, narrow linewidth output from an injection locked high power AlGaAs laser diode array,” Opt. Lett. 19(21), 1741-1743 (1994); Jian Liu, Stable and high speed full range laser wavelength tuning with reduced group delay and temperature variation compensation, Patent Application Number 10/337081, January 2002; and M. Auerbach, et al., “10 W widely tunable narrow linewidth double clad fiber ring laser,” Optics Express 10(2), 139-144 (2002).
In the meantime, continuously frequency-tunable single frequency lasers with linewidth in the order of kHz are important to coherent optical communications, coherent laser radars, optical sensing, test and measurement, and laser tracking of flying objects. For these reasons, there are urgent demand to develop a laser system that the wavelength can be continuously tuned and randomly accessed with excellent wavelength accuracy, stability, and linewidth.
A co-pending application Ser. No. 10/337,081 filed by the Applicant of this invention is hereby incorporated by reference. In this co-pending Application, a single frequency fiber laser to provide laser output of sharp and stable highly defined frequency is disclosed. Meanwhile, there is still a need to provide further improvement on the continuously tunable single frequency lasers with fine linewidth tuning accuracy.
Therefore, a need still exists in the art of fiber laser source design and manufacture to provide a new and improved configuration and method to provide continuously frequency-tunable single frequency lasers with linewidth in the order of kHz such that the above discussed difficulty may be resolved.